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Does Marlin have a reason to write to the SDcard? I guess it must, otherwise the card wouldn't be corrupted.

I do believe that UM2's support for SDHC (i.e. anything >2GB) is flakey or non-existant. I had the printer regularly locking up while attempting to read an 8GB SDHC card, when I reverted to a 2GB SD card it was reliable again. I use that same 8GB card a lot, so I know there is nothing wrong with it. Speaking as am embedded developer myself, including support for SD and SDHC, I'm suspecting that the size of the FAT table may be a problem with the larger cards, depending on how it's processed.

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Does Marlin have a reason to write to the SDcard? I guess it must, otherwise the card wouldn't be corrupted.

I do believe that UM2's support for SDHC (i.e. anything >2GB) is flakey or non-existant. I had the printer regularly locking up while attempting to read an 8GB SDHC card, when I reverted to a 2GB SD card it was reliable again. I use that same 8GB card a lot, so I know there is nothing wrong with it. Speaking as am embedded developer myself, including support for SD and SDHC, I'm suspecting that the size of the FAT table may be a problem with the larger cards, depending on how it's processed.

Marlin has write functions, but they are never called in normal operation. You pretty much have to use custom commands or modify the firmware to write to the card.

As for SDHC, or SD. SDHC can work, but not all manufacturers implement the SPI protocol of SDHC cards correctly, causing problems with some brands. And even within the same brand there are differences. But the card that comes with the UM2 or UMO is very well tested, so that one should work fine.

If you want to know how messy SD card land is. Just look at this compatibility list from another project:

I have a card that gave random problems with Marlin. Made a 1:1 dump, tested the whole SD code in simulation on my PC, no problems then. So I suspect the problem is more with the actual cards then the code.

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Yes, I know about the variable results with sdcards from my own SPI bit banging code, which is why all the cards I use myself are by SanDisk (though even they moved their fab lines to China recently, which has made their timing much less predictable IMO). For clients we only recommend SanDisk and Kingston.

As I said above, I'm quite sure the (SanDisk) 8GB card I was using is a good'un, so my guess is that the firmware just doesn't like 8GB cards. Have you not seen the same thing?